The present invention relates to an absorber with a radiation-selective coating comprising a metal substrate, a diffusion barrier layer, a metallic reflective layer, a cermet layer and an anti-reflective layer. The invention also relates to a method of making an absorber with a radiation-selective absorber coating, in which a metal absorber is polished, is adjusted to a surface roughness at ra<0.3 μm and a diffusion barrier layer, a metallic reflective layer, a cermet layer and an anti-reflective layer are applied to it.
Selective absorber coatings, which are characterized by a high solar absorption coefficient and low thermal emission, are used in thermal solar energy applications for radiation conversion.
Predominantly thin layer systems based on cermet (ceramic-metal mixture) are used, which are produced by vapor deposition or sputtering. The structure of this sort of layer system starting from the substrate surface and progressing to the exterior comprises of the following sequence of layers: a metallic reflective layer, cermet layer and anti-reflective layer.
The metallic reflective layer usually comprises a metal that is highly reflective in the infrared range, such as copper, aluminum or molybdenum. The cermet layer usually comprises an oxide, such as Al2O3, SiO2, which is embedded in a metal, such as Mo. The metal fraction is designated the filling factor.
The anti-reflective layer comprises a pure oxide, such as e.g. SiO2 or Al2O3.
In order to guarantee good adherence of the coating to the substrate, such as copper, stainless steel or glass, additional adhesive layers are used. These adhesive layers comprise chiefly chromium or are chromium-based layers.
Operating temperatures between 300° C. and 600° C., which especially occur in solar concentrating thermal systems, depend on diffusion processes within the absorber layer system and through the absorber layer system. These diffusion processes act negatively on the performance of the entire system.
The following effects are distinguished: The diffusion of elements from the substrate into the absorber coating, which causes a change in the layer properties. For example, in the case of a steel substrate iron diffuses into the layer system.
With vacuum components, such as pipe collectors or receiver pipes for a parabolic gutter or trough, diffusion processes can lead to loss of the vacuum. Because of that the output is reduced. Gas can issue from the substrate or can diffuse through the pipe and through the coating into the vacuum from the heat exchanger passing through the pipe. For example, H2 diffusion through the steel substrate or through the stainless steel substrate is known as one diffusion process.
Absorber pipes made from stainless steel, which are arranged in an evacuated glass tube, are known from “Solar selective absorber coating for high service temperatures, produced by plasma sputtering” by M. Lanxner and Zvi Elgat, SPI″E Vol. 1272, in Optical Materials Technology for Energy Efficiency and Solar Energy Conversion IX (1990), pp. 240 ff. These pipes, which absorb solar energy, conduct a heat exchanger liquid, whose energy is converted into electrical current. The absorber surface has a temperature of 350 to 400° C.
In order to prevent diffusion effects between substrate and absorber coating from occurring, a diffusion barrier layer of Al2O3 is provided. Before the Al2O3 layer is sputtered on, the metal surface of the pipe is polished, so that the average roughness is less than 0.2 μm. After polishing a cleaning process occurs in vacuum under heat transfer.
However diffusion barrier layers from sputtered materials have the disadvantage that they have a porous structure, so that their effectiveness in preventing diffusion is thereby reduced.